With a microscope, after removing the packaging of the chip (with nitric acid, in this case). The readme in the linked repo goes into detail both on how they produced the image used, and how the image is used to extract the ROM.
The ROM itself is 256 bytes, and has been previously extracted using a similar technique. The last thing it does on boot (before moving on to the cartridge’s entry point, which is adjacent) is remove itself from the system’s address space, so special techniques like this are needed to extract it (can’t just do it from normal Game Boy code).
The ROM itself is 256 bytes, and has been previously extracted using a similar technique. The last thing it does on boot (before moving on to the cartridge’s entry point, which is adjacent) is remove itself from the system’s address space, so special techniques like this are needed to extract it (can’t just do it from normal Game Boy code).